The present invention relates to a novel process for performing oxidation reactions, especially those where the direct contact of reactants presents an explosion hazard. The invention overcomes this risk by dissolving a feed component and an oxidizing agent (e.g. oxygen gas) in solution prior to reaction. The process is especially useful for the direct contacting of hydrogen and oxygen to yield hydrogen peroxide.
Currently the most widely practiced industrial scale production method for hydrogen peroxide is an indirect reaction of hydrogen and oxygen employing alkylanthraquinone as the working material. In a first catalytic hydrogenation step, the alkylanthraquinone, dissolved in a working solution comprising organic solvents (e.g. di-isobutylcarbinol and methyl naphthalene), is converted to hydroalkylanthraquinone. In a separate autooxidation step, this reduced compound is oxidized to regenerate the alkylanthraquinone and yield hydrogen peroxide. Subsequent separation by aqueous extraction, refining, and concentration operations are then employed to give a merchant grade product.
Overall, this indirect route to H2O2 formation, whereby a carrier medium is reduced and then oxidized, adds complexity and requires high installation and operating costs. One notable drawback is the significant solubility of the alkylanthraquinone in the aqueous extraction medium used to separate the hydrogen peroxide product. This promotes loss of working solution and leads to contamination of the hydrogen peroxide product with organic species that, when the hydrogen peroxide is concentrated to levels suitable for transport, are reactive with it. A second problem relates to the solubility of the aqueous extraction solution in the alkylanthraquinone working solution. When wet working solution is separated from the aqueous phase for recycle to the indirect oxidation stage, residual aqueous phase xe2x80x9cpocketsxe2x80x9d within the organic solution provide regions for hydrogen peroxide product to concentrate to the extent of becoming hazardous.
Considerably more simple and economical than the alkylanthraquinone route is the direct synthesis of hydrogen peroxide from gaseous hydrogen and oxygen feed streams. This process is disclosed in U.S. Pat. No. 4,832,938 B1 and other references, but attempts at commercialization have led to industrial accidents resulting from the inherent explosion hazards of this process. Namely, explosive concentrations of hydrogen in an oxygen-hydrogen gaseous mixture at normal temperature and pressure are from 4.7-93.9% by volume. Thus the range is extremely broad.
It is also known that dilution of the gaseous mixture with an inert gas like nitrogen scarcely changes the lower limit concentrations, on an inert gas-free basis, of the two gases. Within normal ranges of pressure variation (1-200 atmospheres) and temperature variation (0-100xc2x0 C.) the explosive range is known to undergo little change. Furthermore, even when these reactants are brought together in a ratio that, in the homogeneous condition, would be outside the flammability envelope, the establishment of homogeneity from pure components necessarily involves at least a temporary passage through the flammability envelope. For these reasons, the explosion risks associated with the direct contacting of hydrogen and oxygen are not easily mitigated
In the area of directly contacting hydrogen and oxygen, some efforts have also been made to contain the reaction in a liquid phase. For example, U.S. Pat. No. 5,925,588 B1 discloses the use of a catalyst having a modified hydrophobic/hydrophilic support to provide optimum performance in an aqueous liquid phase. Also, U.S. Pat. No. 6,042,804 B1 discloses dispersing minute bubbles of hydrogen and oxygen into a rapidly flowing acidic aqueous liquid medium containing a catalyst Unfortunately, however, the hydrogen and oxygen reactants are only slightly soluble in the aqueous reaction solvents disclosed in these references.
Other references, namely U.S. Pat. No. 4,336,240 B1 and U.S. Pat. No. 4,347,231 B1 disclose two-phase reaction systems with a homogeneous catalyst dissolved in an organic phase. As mentioned in the former of these two references, homogeneous catalyst systems in general suffer from drawbacks that are a deterrent to their commercial use. The adverse characteristics include poor catalyst stability under reaction conditions, limited catalyst solubility in the reaction medium, and low reaction rates for the production of hydrogen peroxide. In addition, a gaseous H2/O2 containing environment above the two-phase liquid reaction system maintains the equilibrium concentrations of these reactants dissolved in the liquid phase. Therefore, this gaseous atmosphere above the reaction liquid must necessarily be outside the flammability envelope, thus greatly restricting the range of potential reactant mole ratios in the liquid phase.
In contrast to the prior art, the present invention overcomes to a large extent the hazards associated with the direct reaction of hydrogen and oxygen by dissolving these reactant gases into a reaction solvent (e.g. perfluorooctane) in which they are highly soluble. The present invention also relies on heterogeneous reaction chemistry. When hydrogen and oxygen are combined, the product hydrogen peroxide migrates into an aqueous phase, also present in the reaction mixture, from which this product is recovered due to its preferential solubility in this phase.
Several advantages over conventional alkylanthraquinone technology are associated with the present invention. The reaction solvent (e.g. perfluorooctane) dissolves water to a very limited extent, typically less than 30 ppm at saturation and under reaction conditions. Likewise, only minute amounts of reaction solvent are dissolved in the aqueous, or product solvent, phase. Furthermore, the present invention avoids a gaseous environment containing any significant quantities of reactants above the liquid reaction system. This is achieved by feeding reactants directly to the reaction solvent and, above the liquid reaction system, sweeping any unreacted components and contaminants with an inert gas such as nitrogen from the reaction environment.
The realization of a commercially viable direct synthesis of hydrogen peroxide provides a considerable cost savings over the above mentioned indirect alkylanthraquinone route. Furthermore, the direct method of the present invention overcomes the inherent explosion hazards associated with contacting hydrogen and oxygen in the gas phase. The cheaper route to hydrogen peroxide disclosed by applicant also favorably impacts the economics of downstream uses, such as in the further reaction of hydrogen peroxide with propylene to form propylene oxide.
While the synthesis of hydrogen peroxide is of primary interest, the present invention is suitable for a number of oxidative and combustive reactions where an explosion potential exists, for example the conversion of ethylene and oxygen to ethylene oxide, as described in U.S. Pat. No. 4,212,772 B1.
The present invention is a process for the liquid-phase oxidation of hydrogen and hydrocarbons that overcomes inherent explosion hazards associated with directly mixing reactants (e.g. hydrogen and oxygen) in the gas phase. Also, the invention is simpler and cheaper than commercially employed indirect oxidation routes, such as those involving the use of an alkylanthraquinone intermediate to facilitate the overall conversion of hydrogen and oxygen to hydrogen peroxide. The present invention is associated with the realization that certain liquids are capable of dissolving oxygen in concentrations significant enough that hydrogen peroxide and other oxidized products can be synthesized at commercially competitive rates in the liquid phase. Furthermore, immiscible solvents for the reactants and products are used as a means of easily extracting oxidized species preferentially into a product solvent. Extraction of the oxidized product may occur within the reactor simultaneously with the oxidation reaction or it may be a separate step after the oxidation reaction. In the former case, a two-phase liquid reaction environment, into which a solid catalyst is dispersed, is preferably used to effect the oxidation reaction of the present invention.
In one embodiment, therefore, the present invention is a process for oxidizing a feed component with an oxidizing agent, the process comprising dissolving the feed component and an oxidizing agent comprising oxygen in a reaction solvent selected from the group consisting of fluorocarbons, chlorofluorocarbons, hydrochlorofluorocarbons, fluorine-substituted oxygenated hydrocarbons, and mixtures thereof, and thereafter reacting the feed component and the oxidizing agent in the presence of a solid oxidation catalyst and under effective oxidation conditions to yield an oxidized product that is preferentially soluble in a product solvent compared to the reaction solvent.
In a preferred embodiment, the present invention is a process as described above where the reaction solvent, the solid oxidation catalyst, and the product solvent are contained in an oxidation reactor, and the reaction solvent and product solvent are present as separate liquid phases.
In a second embodiment, the present invention is a process for producing an oxidized product, where the process comprises dissolving a feed component and an oxidizing agent in a reaction solvent selected from the group consisting of fluorocarbons, chlorofluorocarbons, hydrochlorofluorocarbons, fluorine-substituted oxygenated hydrocarbons, and mixtures thereof. The process further comprises reacting the dissolved feed component and oxidizing agent in a reaction zone under effective oxidation conditions and in the presence of a solid oxidation catalyst to yield an oxidation product. The process further comprises extracting the oxidation product into a product solvent in which the oxidation product is preferentially soluble. The process further comprises separating the oxidation product and residual amounts of the reaction solvent in the product solvent from the product solvent to yield a regenerated product solvent, a purified oxidation product, and a recovered reaction solvent and recycling the recovered reaction solvent to the reaction zone.
In a preferred embodiment, the present invention is a process as described in the second embodiment, where the steps of dissolving, reacting, and extracting occur within the reaction zone containing the reaction solvent and product solvent as separate liquid phases.
These and other embodiments will be clarified in the following detailed description of the invention.